As a technology according to a head-up display device, in Patent Literature 1, there is disclosed a configuration of “The head-up display device is equipped with: a transmission type liquid crystal display panel; a backlight radiating light to the back of the liquid crystal display panel; and a projection optical system enlarging and projecting an image displayed on the liquid crystal display panel. The projection optical system comprises a relay lens and a projection lens (ocular optical system). The relay lens is constituted to efficiently use telecentric display light by satisfying some conditions, and forms a real image by enlarging the image displayed on the liquid crystal display panel. The projection lens enlarges the real image further and projects it to a windshield of an automobile to display the virtual image for a driver (excerption of the abstract)”.
In the head-up display device of this Patent Literature 1, values of various measuring instruments such as the speed meter, tachometer, water temperature meter, fuel gage, and the like are displayed by a virtual image at 2 m forward of the driver. Thus, because the difference between the visual direction of viewing the values of the various measuring instruments by a virtual image and the visual direction of the foreground viewed by the driver becomes small, the time taken for moving the line of sight between these 2 visual directions can be shortened.
Also, because the distance to the virtual image (2 m forward) is closer to the distance to the foreground viewed by the driver compared to the distance of directly viewing various measuring instruments and the like, the time required for focusing the eyes between a state where the eyes are focused on an object in the foreground and a state where the eyes are focused on the virtual image can be also shortened.
With these 2 advantages, it can be expected to improve safety in driving an automobile by the head-up display device.
However, because the foreground of the driver is not a two-dimensional plane surface but a three-dimensional space, there is a problem that, when information according to an object having a different distance from the eyes of the driver is displayed on the same virtual image plane, the line of sight distance of the object and the displayed information differs, and it takes time further for focusing the eyes.
Therefore, in Patent Literature 2, there is disclosed a configuration of “By disposing the first screen and the second screen at positions shifted from each other on the optical path, respective virtual images are formed at different distances from the driver, and by disposing a variable focus lens, an image is focused in a state where image light is focused by the first screen or the second screen. As the variable focus lens, separately from a projection lens, a liquid lens for changing the focal length and a concave mirror configured to be capable of changing the laser beam and the curvature are driven (excerption of the abstract)”.
According to the head-up display device of this Patent Literature 2, because the virtual image can be formed at different distances from the driver, a virtual image plane close to the distance of an object in the foreground of the driver can be selected. Therefore, the time required for focusing the eyes in a state where the eyes are focused on the object in the foreground of the driver and a state where the eyes are focused on the virtual image can be shortened further.